An optical transceiver module is an optical communications device that has a transmit (TX) portion and a receive (RX) portion. The TX portion includes a laser driver circuit and at least one laser diode. The laser driver circuit outputs an electrical drive signal to each respective laser diode to cause the respective laser diode to be modulated. When the laser diode is modulated, it outputs optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system of the optical transceiver module focuses the optical signals produced by each respective laser diode into the end of a respective transmit optical fiber held within an optical connector module that connects to the optical transceiver module.
The RX portion of the optical transceiver module includes at least one receive photodiode that receives an incoming optical signal output from the end of a respective receive optical fiber held in the optical connector module. The optics system of the transceiver module focuses the light that is output from the end of each receive optical fiber onto the respective receive photodiode. The respective receive photodiode converts the incoming optical signal into an electrical analog signal. An electrical detection circuit, such as a transimpedance amplifier (TIA), receives the electrical signal produced by the receive photodiode and outputs a corresponding amplified electrical signal, which is processed by other circuitry of the RX portion to recover the data.
Some optical transceiver modules have a single laser diode in the TX portion and a single photodiode in the RX portion for simultaneously transmitting and receiving optical signals over transmit and receive fibers, respectively, of transmit and receive optical cables, respectively. The ends of the transmit and receive cables have optical connector modules on them that are adapted to plug into transmit and receive receptacles, respectively, formed in the optical transceiver module. These types of optical transceiver modules are often referred to as pluggable transceiver modules. Small form-factor pluggable (SFP) and SFP+ transceiver modules are examples of pluggable optical transceiver modules. Parallel optical transceiver modules of the type described above may also be configured as pluggable optical transceiver modules, but may also be configured as mid-plane mounted optical transceiver modules that mount to a surface of a circuit board.
Some optical transceiver modules have multiple laser diodes in the TX portion and multiple photodiodes in the RX portion for simultaneously transmitting and receiving multiple optical signals. In these types of optical transceiver modules, which are commonly referred to as parallel optical transceiver modules, the transmit fiber cables and the receive fiber cables have multiple transmit optical fibers and multiple receive optical fibers, respectively. The cables are typically ribbon cables having ends that are terminated in an optical connector module that is configured to be plugged into a receptacle of the optical transceiver module.
Typically, pluggable optical transceiver modules, such as the SFP and SFP+ optical transceiver modules, for example, are designed to be inserted into cages. The pluggable transceiver modules and the cages have locking features disposed on them that allow the transceiver modules to mate with and interlock with the cages. The pluggable transceiver modules typically include latch lock pins that are designed to be received in latch openings formed in the cages. In most pluggable optical transceiver module designs, the area around the latch lock pin constitutes an EMI open aperture that allows EMI to escape from the transceiver module housing. The Federal Communications Commission (FCC) has set standards that limit the amount of electromagnetic radiation that may emanate from unintended sources. For this reason, a variety of techniques and designs are used to shield EMI open apertures in transceiver module housings in order to limit the amount of EMI that passes through the apertures. Various metal shielding designs and resins that contain metallic material have been used to cover areas from which EMI may escape from the housings. So far, such techniques and designs have had only limited success, especially with respect to optical transceiver modules that transmit and receive data at very high data rates (e.g., 10 gigabits per second (Gbps)).
For example, EMI collars are often used with pluggable optical transceiver modules to provide EMI shielding. The EMI collars in use today vary in construction, but generally include a band portion that is secured about the exterior of the transceiver module housing and spring fingers having proximal ends that attach to the band portion and distal ends that extend away from the proximal ends. The spring fingers are periodically spaced about the collar. The spring fingers have folds in them near their distal ends that cause the distal ends to be directed inwards toward the transceiver module housing and come into contact with the housing at periodically-spaced points on the housing. At the locations where the folds occur near the distal ends of the spring fingers, the outer surfaces of the spring fingers are in contact with the inner surface of the cage at periodically spaced contact points along the inner surface of the cage.
The amount of EMI that passes through an EMI shielding device is proportional to the largest dimension of the largest EMI open aperture of the EMI shielding device. Therefore, EMI shielding devices such as EMI collars and other devices are designed to ensure that there is no open aperture that has a dimension that exceeds the maximum allowable EMI open aperture dimension associated with the frequency of interest. For example, in the known EMI collars of the type described above, the spacing between the locations at which the distal ends of the spring fingers come into contact with the inner surface of the cage should not exceed one quarter wavelength of the frequency of interest that is being attenuated. Even greater attenuation of the frequency of interest can be achieved by making the maximum EMI open aperture dimension significantly less than one quarter of a wavelength, such as, for example, one eighth or one tenth of a wavelength. However, the ability to decrease this spacing using currently available manufacturing techniques is limited. In addition, as the frequency of optical transceiver modules increases, this spacing needs to be made smaller in order to effectively shield EMI, which becomes increasingly difficult or impossible to achieve at very high frequencies.
In parallel optical transceiver modules, the optical cables that carry the fibers are typically ribbon cables in which the fibers are arranged side-by-side in a 1×N array, where N is the number of fibers of the ribbon cable. Thus, the transmit fibers are arranged in one 1×N fiber array in one ribbon cable and the receive fibers are arranged in another 1×N array in another ribbon cable. Typically, the ribbon cables are placed one on top of the other such that a 2×N array of fibers enter the optical connector module through a gap formed in the nose of the optical connector module. This gap constitutes an EMI open aperture that is much larger than the maximum allowable EMI open aperture dimension of the optical transceiver module, particularly at high bit rates. Consequently, unacceptable amounts of EMI may escape from the optical transceiver module through the gap.
One technique that is sometimes used to provide EMI shielding at the gap in the optical connector module involves placing a metal EMI shielding device in the nose of the optical connector module surrounding the gap such that the fibers pass through the EMI shielding device. While such shielding devices are relatively effective at preventing EMI from passing through regions in the housing immediately adjacent the gap, they are totally ineffective at preventing EMI from passing through the gap itself, which is filled only with the fibers and air. Of course, the fibers and the air are transparent to EMI.
In general, all of the current techniques of providing EMI shielding in optical transceiver modules attempt to ensure that there are no EMI open apertures that have dimensions that exceed the maximum allowable EMI open aperture dimension. As indicated above, as the frequencies or bit rates of optical transceiver modules continue to increase (i.e., wavelengths continue to decrease), it becomes extremely difficult or impossible to effectively implement these types of solutions. Accordingly, a need exists for an EMI shielding device and a method that do not rely solely on such techniques to provide effective EMI shielding in optical transceiver modules.